Ink jet imaging devices eject liquid ink from printheads to form images on an image receiving member. The printheads include a plurality of ink jets that are arranged in some type of array. Each ink jet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The frequency and amplitude of the firing signals correspond to the selective activation of the printhead actuators. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member to form an ink image that corresponds to the digital image used to generate the firing signals.
Throughout the life cycle of these ink jet imaging devices, the image generating ability of the device requires evaluation and, if the images contain detectable errors, correction. Missing ink jets or weak ink jets are an error condition that affects ink image quality. A missing ink jet is an ink jet that does not eject an ink drop in response to a firing signal. A weak ink jet is an ink jet that responds intermittently to a firing signal or that responds by ejecting ink drops having a mass that is less than the ink drop mass corresponding to the characteristics of the firing signal for the ink jet. Systems and methods have been developed that compensate for missing or weak ink jets, but the missing or weak ink jets must be detected before these systems and methods can be activated.
Detection of missing and weak ink jets is made difficult by the surface characteristics of the image receiving member. In some ink jet imaging devices, the ink is ejected onto a media substrate and the ink image is fused onto the sheet. The sheet is ejected and then imaged by illuminating the surface of the sheet and generating an electrical signal that corresponds to the intensity of the light reflected from the surface. The signal is generated by a photo detector that is positioned to receive light reflected from a small portion of the image surface. By arranging a plurality of photo detectors across the width of a media sheet, the entire width of the sheet may be used to generate reflected light received by the photo detectors. The responses of the photo detectors produce a digital image corresponding to the ink image on the media sheet. The ink drops on the sheet reflect light at an intensity that is different than the positions on the sheet that do not have ink on it. In other ink jet imaging devices, the ink is ejected onto a rotating image member, such as an anodized drum or an endless belt, and the ink image formed on the belt is transferred to a media sheet by forming a nip with a pressure roller and synchronizing the delivery of the media sheet to the nip to coincide with the arrival of the image on the rotating image member at the nip. In these types of devices, the ink image may be imaged from the media sheet to check for missing or weak jets. Alternatively, the ink image on the rotating imaging member may be illuminated and the reflected light used to generate a digital image that corresponds to the ink image on the rotating image member.
Evaluating a digital image produced by illuminating an image member can be difficult because the surface of the image member may generate noise in the digital image. For example, the random structure in a media sheet or anodized drum may reflect light away from a photo detector and emulate the amount of light reflected by an ink drop, which absorbs light. Consequently, systems and methods analyzing digital images of ink images on an image substrate to detect missing or weak ink jets need to be able to distinguish structure in the image substrate from the absorption of light by an ink drop. Another source of noise is the location of ink drops in the test pattern formed on an image member. The image of the test pattern on the image member may be captured by a plurality of light sensors, such as photo detectors, arranged linearly across the image receiving member. If an ink drop being imaged lies on a boundary of a field of vision for two adjacent light sensors, then the light absorbed by the ink drop may not be fully detected by either light sensor.
Before an ink jet imaging device leaves a manufacturing facility, the device should be tested to determine whether the printhead has a number of missing or weak jets that would adversely affect image quality. Additionally, ink jets in the printheads of an ink jet imaging device may begin to exhibit missing or weak ink jet characteristics. These changes arise because the device and its environment may experience temperature instabilities, dust, or other debris, which may cause components of the device to shift or operate unreliably. These conditions may cause the intrinsic performance of the device to change reversibly or irreversibly. Consequently, the ink jets of the printheads in an ink jet imaging device require evaluation at various intervals during the operational life of the device to detect changes in the performance of the ink jets. Sometimes these evaluations and adjustments are made at time or usage intervals, while at other times the adjustments are made during service calls made by trained technicians. Consequently, the ability to detect missing and weak ink jets in an ink jet imaging system is important.